Oil-vapor diffusion vacuum pump



25heets-Sheet 1 Filed Nov. 5, 1964 Feb. 7, 1967 M. NICOLAS 3,302,864

OIL-VAPOR DIFFUSION VACUUM PUMP Filed Nov. 5, 1964 2 Sheets-Sheet 2United States Patent Ofiice 3,362,864 Patented Feb. 7, 1967 3,302,864OIL-VAPOR DIFFUSION VACUUM PUMP Michel Nicolas, Bievres, France,assignor to Compagnie Industrielle Francaise des Tubes Electroniques,Courbevoie, Seine, France Filed Nov. 5, 1964, Ser. No. 409,265 Claimspriority, application France, Nov. 8, 1963, 953,126, Patent 1,382,330 2Claims. (Cl. 230-101) This invention relates to improvements in sea-ldevices for use in vacuum pumps, operating according to the oilvapordiffusion principle, .to permit return to the boiler of the oil.condensed in the pump body while preventing any flow in the oppositedirection of oil vapor generated in the boiler. Thus, the vapor,according to the conventional principle of operation of these vacuumpumps, will necessarily flow through the intermediate member or ejectorfrom which this vapor is caused to diffuse into the pump body beforecondensing on the cooled wall of said body.

In conventional oil-vapor diffusion vacuum pumps this return flow ofcondensed oil to the boiler is effected in most instances through a pipeimmersed in the liquid-oil charge or" the boiler; therefore, this devicehas a considerable length in proportion to the total length of the pump,thus increasing inasmuch the over-all dimensions of the pump andconsequently its weight. Now, for various reasons to be set forthpresently it is particularly desirable to reduce both the weight and theover-all dimensions of oil-vapor diffusion vacuum pumps notably in thespecific field of automatic pumping units such as those used in themass-production of electron tubes and other sealed enclosures forvacuumizing and degasifying same.

Such automatic units, mainly of the rotary turret type, may comprise arelatively high number of working positions or stations (for example 16,24, 30, 36 or more), each position or station having its diffusion pump,this requirement being of primary interest for combining highqualityproducts with high pumping rates.

The use of a diffusion pump at each working station is obviouslyattended (for the same search for higher production rates and higherpumping efiiciencies) by the necessity of providing a direct connectionbetween said pump and the enclosure to be vacuumized; this requirementis met by mounting at each working position the diffusion pump in directalignment with the nozzle through which the enclosures loaded on theunit are vacuumized, these enclosures being disposed on the outerperiphery of the circular rotary turret in order not only to have themaximum relative peripheral spacing between them but also for obviousreasons of convenience in operating the machine, notably as to loadingand unloading.

Since it is essential that the angular movement corresponding to thepositioning of the unit takes place as rapidly as possible, in order toreduce the resulting idle period, it is necessary that the higher thepumping rate, the lower the moment of inertia of the turret.

Now, due to the compulsory mounting of the diffusion pumps at theoutermost periphery of the rotary turret the greater part of the turretweight is transferred to its periphery; therefore, the lighter thediffusion pumps equipping the pumping units, the lower the moment ofinertia of the turret.

On the other hand, the mechanical construction of these units and theiroperation are greatly facilitated by lowering the working level of theturret (thus providing a more compact assembly). Furthermore, an easieraccess can thus be had to the turret plate for loading and unloading theenclosures to be vacuumized and for adjusting the working stations;therefore, the diffusion pumps which, for the aforesaid reasons ofmounting in direct alignment with the enclosure pumping nozzles, canonly be mounted underneath said plate, must be relatively short (whichis consistent with the search for lighter weight for meeting theabove-mentioned requirement of reduction in the moment of inertia of theturret).

Finally, the relatively high number of working stations and therefore ofpumping units leads to the use of an electric commutator for feeding theheating elements of these pumps which has definitely prohibitivedimensions if the heating power of these elements is relatively high;thus, to reduce this heating power within reasonable limits thediffusion pumps to be mounted on the automatic pumping units mustnecessarily have a very high thermal efficiency.

It is the essential object of this invention to provide a device adaptedto impart to oil-vapor diffusion pumps the three advantageous featuresset forth hereinabove.

It is known in oil-vapor diffusion pumps to take advantage of theposition of the ejector concentric to the pump body for providing anannular gap or space between its outer wall and the inner wall of thepump body. Thus, the oil having condensed will drip along the cooledwall of the pump body and accumulate in the lower portion of the annularspace in the form of a liquid ring which, being retained by surfacetension in said space, constitutes a kind of liquid seal therein, thuspreventing the diffusion of the oil vapor from the boiler to the workingchamber, or, in other words, forcing this oil vapor to flow through theejector.

However, the seal thus obtained in the form of a single ring of liquidoil cannot be effective unless this ring is of relatively great heightor thickness, for example of the order of one inch. Thus, although thisdimension is acceptable in the case of relatively large diffusion pumpsnot limited by maximum over-all dimensions, it cannot be contemplated inthe case of pumps to be mounted on automatic pumping units due to theessential requirements set forth hereinabove. On the other hand, themagnitude of the total area of the registering metal surfaces on eitherside of the oil ring implies, for the ejector, a loss of heat which isproportional to said area. This loss of heat occurs as a matter of factthrough the hot oil towards the cooled pump body. As in the precedingcase, although this loss of heat is relatively immaterial in the case ofa diffusion pump of relatively high heating power capacity, the samerelative loss is on the other hand extremely detrimental in the case ofpumps having desirably (as already explained hereinabove) the bestpossible thermal efficiency for a minimum heating power.

Another feature characterizing this invention is that it permitsreducing not only the heightof the ring of condensed oil but also thesurface area of the ejector and pump bodies. The height of the oil ringis reduced by splitting the ring into at least two rings or sub-rings,and the surface area is reduced by forming the ejector body with asplined or castellated section by machining at least three collarsregularly spaced in the axial direction, these collars having differentdiameters so as to leave gradually increasing gaps between themselvesand the inner wall of the pump body, two adjacent collars formingtherebetween an annular cavity. Under these conditions the deviceconsidered as a whole has at least two such annular cavities wherein theoil condensing along the pump body flows towards the boiler toconstitute a seal in the form of said liquid rings or sub-rings.

The relative spacing of said collars is determined by the trial anderror method so that between two adjacent collars or rings (with dueconsideration for the volumes occupied by these rings) there is anoil-free space, the above-defined annular cavities thus forming behindsaid rings what may be termed expansion chambers, whereby, throughoutthe height of the zone bounded by said rings, a pressure gradient iscreated which, in conjunction with the temperature drop produced at thislevelas a I consequence of the presence of condensed oil, causes any oilvapor molecule tending to escape from the boiler along the pump body andthe ejector body to be compulsori-ly condensed at least in the secondcavity (as proved experimentally by observing the phenomenon in a pumpprovided with the deviceconstituting the subject-matter of thisinvention but having a glass and therefore transparent body) so that theassembly actually corresponds to the desired sealing device, theprovision of a third cavity (by machining a fourth collar on the ejectorbody) being merely an additional safety measure; therefore, the fourthcollar or ring is considered just as supplementing or doubling the thirdone and may have exactly the same shape and dimensions, thus providingthe same gap between its outer periphery and the inner wall of the pumpbody, whereas the second and first collar (counted in this order fromthe boiler outwards or upwards) form with the inner wall of the bodygaps of gradually decreasing width.

As the volumes ofthe successive liquid rings depend on the oilviscosity, the geometrical shape of the device taken as a whole, issubordinate to the quantity of oil utilized. In the case of thegeometrical arrangement contemplated by the invention the dimensions ofthe gaps left between each collar and the inner wall of the pump bodyare respectively 0.03" in the case of the fourth and third rings orcollars, 0.02" in the case of the second collar, and 0.012" in the caseof the first collar, the height of the zone covered by these fourcollars being A and, corresponding to a relative spacing of 0.20 betweenadjacent rings or'collars and a thickness of 0.04" for each collar,these dimensions being given by way of example, of course,without'limiting in any way the present invention.

Apart from its reduced length the device of this invention, which iseasy to manufacture and maintain, is characterized by the followingadvantageous features:

(a) During its flow the oil is retained in the proximity of the primarypumping orifice and therefore subjected to a temperature ranging from160 C. (320 F.) to 190 C. (374 F.); therefore, this oil is partiallyfreed by fractional distillation of any occluded impurities;

(b) The oil will drip not along the ejector (which it would cool whilere-evaporating) but under a gutterforming rib proved to this end at thejunction of the pump body with the boiler; (c) The seal consisting ofthe liquid oil kept between the ejector and the outer wall of the pumpis essentially, due to its nature, a poor heat conducting substance;therefore, no undesired cooling of the ejector is likely to be producedthereby.

These two last advantageous features play to a considerable extent anessential part in the improvement of the thermal efilciency of the pumpsprovided with the device of this invention, thus imparting to thesepumps the third desired property among the three ones listed andexplained hereinabove.

In order to afford a clearer understanding of this invention and of themanner in which the same may be carried out in practice, reference willnow be made to the accompanying drawings illustrating diagrammaticallyby way of example a typical form of embodiment of this inventron.

- In the drawings:

FIG. 1 is a longitudinal or axial section showing a pump assemblyequipped with the device constituting the subject-matter of thisinvention;

FIG. 2 is another axial sectional view showing on a larger scale thepump portion constituting the aforesaid device; and

FIG. 3 is a fragmentary enlarged View of the pump portion of FIG. 2provided with a seal device according to the invention and illustratesthe formation Of th $63 by the seal device.

Thepump body 1 with its cooling jacket 2 carries at its lower portion aboiler 3 and at its upper portion a flange 4 for connecting the pump toa pipe, duct or enclosure to be vacuumized and not shown. A nozzle 5permits connecting the pump to a primary pump (not shown) for primingand maintaining the preliminary vacuum. An ejector 6 (with its steamdeflection cones or jet means such as 7 and 8) is centered on a bottomplate 9 of the boiler 3 by means of a simple central rod arrangement 10ermitting a convenient assembling and disassembling for maintenancepurposes. A heating element 11 (consisting for example of a shieldedresistance of adequate configuration contacting the outer surface ofboiler plate 9 by means of a fiat face increasing considerably the heattransfer from said heating element and the boiler), and a protectivecase 12 are both secured to theboiler by means of a screw-threaded rod13. A set of collars or rings 14 machined on the outer surface of theejector provides in conjunction with the pump body and the ejector bodythree annular cavities constituting as many expansion chambers C C and CThe configuration of these rings or collars is shown more in detail inFIGS. 2 and 3 together with the dimension of the corresponding radialannuli or gaps d, c, b, a, formed between each collar and the inner wallof the pump body. The actual values of these gaps, which, de creasetowards the boiler have already been given hereinabove by way of examplein the case of a specific oil grade used by the applicant, namelysilicone fluids Si 703 and 704. v v

A drip rib 15 is provided at the lower end of the pump body, around theaperture thereof opening into the boiler, whereby the condensed oil willreturn to the boiler from this rib, that is, from the pump body, insteadof from the lowermost ring of the ejector, thus avoiding, as already setforth hereinabove, the cooling of the ejector by re,- evaporation of oilin this zone and therefore further increasing the thermal efficiency ofthe assembly. Besides, the factthat. the. ejector, due to its specificmounting, is completely independent of the pump body (as contrasted withpumps of relatively moderate overall dimensions of prior artarrangements) further improves the thermal efficiency of the assembly.

Moreover, it will be seen that with the arrangement contemplated hereinthe pumps thus equipped are characterlzed by an improvement of the limitvacuum, this improvement being apparently ascribab-le to a kind offractional distillation adapted to promote the separation of the gaseousmolecules included in the oil used in the pump.

An alternate form of embodiment of this invention consists in formingthe annuli on the inner wall of the pump body instead of on the outerwall of the ejector body which in this case is smooth, the same relativespacings and gaps being provided between the corresponding portions ofsaid annuli registering with the outer wall of the ejector; however, tosimplify the machining operations the preferred disposal is thatdescribed hereinabove and shown in the attached drawings.

Of course, the device of this invention, while developed and appliedherein to pumps of moderate size and therefore of moderate capacity isalso applicable to largecapacity pumps.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

What I claim is:

1. In a vapor diffusion vacuum pump having a boiler containing andheating an organic working fluid to a vapor, an ejector comprising aninner tubular element mounted in communication with said boner forreceiving vapor of the working fluid and means defining vapor jet meansfor developing a vacuum with said ejector in operation, an outer wallcircumferentially of said inner tube extending axially of said innertube and spaced outwardly therefrom defining an annular space incommunication with said boiler for returning condensed vapor of saidworking fluid to said boiler, the improvement which comprises seal meansin said space to preclude working fluid in a vapor phase from saidboiler entering said annular space and allowing working fluid in acondensed state to return to said boiler, said seal means comprising aplurality of collars disposed axially spaced in said tubular spaceextending inwardly defining in said space annular gaps of differentdimensions between the inner tube and said outer peripheral walleffective to entrap condensed working fluid in a liquid phase to formrespective seal rings of liquid working fluid at each of said gaps, andsaid gaps being of different respective dimensions and the diflFerentdimensions thereof decreasing in a direction toward said boiler.

References Cited by the Examiner UNITED STATES PATENTS 2,404,022 7/1946Alexander et al. 230-101 2,585,139 2/1952 Lawrence et al. 23010l FOREIGNPATENTS 1,032,470 6/ 1958 Germany.

475,062 10/1937 Great Britain.

DONLEY J. STOCKING, Primary Examiner.

MARK M. NEWMAN, Examiner.

W. L. FREEH, Assistant Examiner.

1. IN A VAPOR DIFFUSION VACUUM PUMP HAVING A BOILER CONTAINING ANDHEATING AN ORGANIC WORKING FLUID TO A VAPOR, AN EJECTOR COMPRISING ANINNER TUBULAR ELEMENT MOUNTED IN COMMUNICATION WITH SAID BOILER FORRECEIVING VAPOR OF THE WORKING FLUID AND MEANS DEFINING VAPOR JET MEANSFOR DEVELOPING A VACUUM WITH SAID EJECTOR IN OPERATION, AN OUTER WALLCIRCUMFERENTIALLY OF SAID INNER TUBE EXTENDING AXIALLY OF SAID INNERTUBE AND SPACED OUTWARDLY THEREFROM DEFINING AN ANNULAR SPACE INCOMMUNICATION WITH SAID BOILER FOR RETURNING CONDENSED VAPOR OF SAIDWORKING FLUID TO SAID BOILER, THE IMPROVEMENT WHICH COMPRISES SEAL MEANSIN SAID SPACE TO PRECLUDE WORKING FLUID IN A VAPOR PHASE FROM SAIDBOILER ENTERING SAID ANNULAR SPACE AND ALLOWING WORKING FLUID IN ACONDENSED STATE TO RETURN TO SAID BOILER, SAID SEAL MEANS COMPRISING APLURALITYOF COLLARS DISPOSED AXIALLY SPACED IN SAID TUBULAR SPACEEXTENDING INWARDLY DEFINING IN SAID SPACE ANNULAR GAPS OF DIFFERENTDIMENSIONS BETWEEN THE INNER TUBE AND SAID OUTER PERIPHERAL WALLEFFECTIVE TO ENTRAP CONDENSED WORKING FLUID IN A LIQUID PHASE TO FORMRESPECTIVE SEAL RINGS OF LIQUID WORKING FLUID AT EACH OF SAID GAPS, ANDSAID GAPS BEING OF DIFFERENT RESPECTIVE DIMENSIONS AND THE DIFFERENTDIMENSIONS THEREOF DECREASING IN A DIRECTION TOWARD SAID BOILER.